1. Field of the Invention
This invention relates to control of gear ratio changes in automatic transmissions. More particularly, the invention pertains to altering, on the basis of variations in engine output torque, vehicle speed and engine speed, where upshifts, downshifts and torque converter bypass clutch engagements and releases are scheduled to occur according to reference schedules of control.
2. Description of the Prior Art
Conventionally, the electronic control system for an automatic transmission is provided with a gearshift schedule that defines conditions of current engine throttle position and vehicle speed at which each gear ratio change is to occur. Generally, data located in look-up tables stored in electronic memory, relating vehicle speed and engine throttle position for each of the gear ratio changes, are used to produce a signal representing the desired gear ratio. When the desired gear ratio is different from the current gear ratio, a command signal is produced by the control to cause a change in state of solenoid-operated shift valves. This changes the state of the planetary gear units and causes the gear ratio to change.
A reference schedule of gearshift boundaries is continually referenced during execution of control algorithms and used to produce an upshift signal and a downshift signal when a boundary of the gearshift schedule is crossed. Gearshifts are made by continually probing computing memory with current values of throttle position and vehicle speed, and determining, on the basis of the stored shift schedules, whether an upshift or downshift is required. Upshift points are determined also on the basis of maximum engine speed when a wide-open throttle condition is detected.
U.S. Pat. No. 4,943,921 describes an electronic control for scheduling gearshifts in an automatic transmission on the basis of variations in ambient barometric pressure. Various functions relating vehicle speed and throttle position determine the occurrence of each upshift and downshift by probing computer memory, in which the gearshift functions reside, with current values of the throttle position and vehicle speed. A reference gearshift schedule is corrected for the difference between the operating barometric pressure and a reference barometric pressure, at which the gearshift schedules are calibrated, to automatically compensate for the effect of barometric pressure variations on the gearshift schedule.